The invention generally relates to ink compositions for ink jet printers. More particularly, the invention relates to utilizing an ultraviolet light or visible light sensitive ink for jet printing.
Ink jet printing has experienced a significant increase in use in recent years displacing other printing methods due to reduced cost and improvements in print speed and print resolution and the ability to print continuously variable data at high speed through computer control. There are two major categories of ink jet printing--"Drop-on-Demand" and "continuous" ink jet printing. For continuous ink jet, a conducting ink is supplied under pressure to an ink nozzle and forced out through a small orifice. Prior to passing out of the nozzle, the pressurized ink stream proceeds through a ceramic crystal which is subjected to an electric current. This current causes a piezoelectric vibration equal to the frequency of the AC electric current. This vibration, in turn, generates the ink droplets from the unbroken ink stream. The ink stream breaks up into a continuous series of drops which are equally spaced and of equal size. Surrounding the jet, at a point where the drops separate from the liquid stream in a charge electrode, a voltage is applied between the charge electrode and the drop stream. When the drops break off from the stream, each drop carries a charge proportional to the applied voltage at the instant at which it breaks off. By varying the charge electrode voltages at the same rate as drops are produced it is possible to charge every drop to a predetermined level. The drop stream continues its flight and passes between two deflector plates which are maintained at a constant potential. In the presence of this field, a drop is deflected towards one of the plates by an amount proportional to the charge carried. Drops which are uncharged are undeflected and collected into a gutter to be recycled to the ink nozzle. Those drops which are charged, and hence deflected, impinge on a substrate traveling at a high speed at right angles to the direction of drop deflection. By varying the charge on individual drops, the desired pattern can be printed.
In a typical "Drop-on-Demand" ink jet printing process, a fluid ink is typically forced under pressure through a very small orifice of a diameter typically about 0.0024 inches in the form of minute droplets by rapid pressure impulses. The rapid pressure impulses are typically generated in the print head by either expansion of a piezoelectric crystal vibrating at a high frequency or volatilization of a propellant within the ink by rapid heating cycles. The piezoelectric crystal expansion causes the ink to pass through the orifice as minute droplets in proportion to the number of crystal vibrations. Thermal ink jet printers employ a heating element within the print head to volatilize a propellant and form droplets in proportion to the number of on-off cycles for the heating element. The ink is forced out of the nozzle when needed to print a spot on a substrate as part of a desired image. The minute droplets may be energized to achieve an electrical charge and deflected as in the continuous ink jet printing. Conventional ink jet printers are more particularly described in U.S. Pat. No. 3,465,350 and U.S. Pat. No. 3,465,351.
Another type of ink jet printing process is an electrostatic ink jet process which employs an electrostatic field to draw the ink through the nozzle to the substrate. Charged ink droplets are drawn to an oppositely charged platen behind the receiving substrate. Such devices have been developed by Technology International Corp. of Boulder, Colo., under the tradename ESIJET.TM..
To operate satisfactorily within an ink jet printer, the ink has many requirements, the ink must exhibit low viscosity values typically below 20 centipoise at 25.degree. C., contain no large particulate matter (typically below 5 .mu.m), and be sufficiently stable so as not to dry and clog the ink jet orifice over short periods of time when not in use. For "continuous" ink jet printing, the ink must be conductive with a resistivity value typically below 10,000 ohms and the unused ink must be recyclable. Secondary factors must also be considered, such as avoiding the generation of flammable vapors during use and minimizing the impact on the environment. Conventional inks which meet these criteria typically do not provide images or indicia with high smear resistance and high chemical resistance. Jet printing inks with improved resistance to handling (smearing) and chemicals are desired.
Inks which provide high smear resistance and high chemical resistance are known. However, these inks are commonly used in other printing methods such as screen printing and lithography techniques for printed circuit boards. These inks typically do not have the low viscosity requirements and/or other requirements of inks used in modem ink jet printers. An example of an ultraviolet light cured ink said to be useful for screen printing and resistant to chemicals and smear is disclosed in U.S. Pat. No. 5,200,438. This ultraviolet ray-curable ink formulation employs a relatively high molecular weight polymer having a rubbery elastomeric structure as a reactive compound.
Other ultraviolet radiation curable inks are known and most comprise a reactive oligomer, a reactive monomer, a photoinitiator, a pigment and optional additives. Of the various types of ultraviolet light curable inks which are known, most find use in screen printing methods, examples being those described in U.S. Pat. No. 5,200,438, discussed above, as well as U.S. Pat. Nos. 5,391,685, 4,680,368 and 5,500,040. A UV curable ink said to be suitable for ink jet printing is described in U.S. Pat. No. 4,258,367. This ink comprises a diazonium compound or derivative thereof as the UV active component. The diazonium compound changes color upon exposure to UV light and becomes visible. These inks do not contain a binder for a pigment or dye which provides resistance to handling (smear) and chemicals.
An etch resistant UV curable ink jet printing ink used to prepare printed circuit boards is described in U.S. Pat. No. 5,270,368. This ink is based on acrylate components and it is said this ink provides excellent adhesion to metal substrates. It is desirable to provide other UV curable ink jet inks with alternative property profiles, such as being well suited for printing on paper and incorporating pigments therein.